The objective of the proposed study is to further our understanding of how mutagens and potential carcinogens exert their effect. In particular, this study will focus on detection of agents whose mutagenic range includes induction of large but submicroscopic deletions and DNA rearrangements. Since there has been no simple test for distinguishing such lesions, it is likely that the contribution of large deletions and rearrangements to the spectrum of mutations induced by mutagenic agents has been underestimated. Among the proposed agents to be tested will be several mutagens, including those commonly associated with induction of base substitution and point deletion. This study will determine whether or not, and to what extent, such agents might also contributes to larger deletions and rearrangements that are not detectable cytologically. The basis for the proposed assay is reversion of cultured cells to a selectable phenotype. The target for reversion is a mouse APRT gene that has been interrupted within an intron by foreign DNA whose presence interferes with gene function. This modified gene is introduced into APRT- deficient human cells as a single-copy integrant. Reversion of Aprt-deficient human cells carrying this construct, to the Aprt+ phenotype requires loss or rearrangement of part or all of the inserted DNA. Once validated, this test should provide a rapid and accurate screen for agents that promote DNA deletion or rearrangement.